I went up to Murray's today for a free diagnostic test. My check engine light came on AFTER I replaced the spark plugs, wires, air filter. The code was "P0305"-Miss fire of the 5th cylinder.

Now I have had this truck for a month. When I bought the truck at auction the light was not on. It's a 2000 Ranger XLT, 4.0 V6 with 112,000 miles on it. Below 1,000 RPM (Basically at idle) the engine shakes like a SOB. Also when I am in reverse the truck shakes too. Soon as I hit 2,500 RPM the shaking stops. I also think my fuel economy sucks because of this problem.

Any suggestions OR has this happened to anyone else? my father-in-law suggested replacing the ignition coil pack and to do a compression test to make sure the piston valves are in good shape.

try swapping the plug with one from another cylnder if the miss is still #5 try swapping the plug wire with another. you can also check the coil pack by pulling the wire off the plug and install an old plug and rest it on the exhaust manifold start the truck and look to see if it is firing.

Sounds pretty simple. Make sure your plug wires are installed properly. Try your old #5 plug wire and see if that new one is bad. Check the plug gap and try one of the old plugs in there.
You may have just switched two of the wires when you installed them.